An electrical discharge machine with an electrode in the form of an electrically conductive wire, referred to as an electrode wire, electrically machines a workpiece into the desired shape by using an electrical discharge generated between the electrode wire and the workpiece by applying a discharge voltage therebetween. The electrode wire is drawn from a wire bobbin by a wire conveying device, and moves through upper and lower wire guides provided in a machining region.
A wire tension setting device is provided between the wire bobbin and the wire conveying device in order to apply a wire tension to the moving electrode wire.
Referring FIG. 3, an example of the wire tension setting device of the prior art is illustrated.
The wire tension setting device of FIG. 3 comprises a brake roller 6 which applies a braking torque to an electrode wire 2. The brake roller 6 is connected to a output shaft 75 of a powder clutch 74, and is rotatable about the output shaft 75. The electrode wire 2 is around the brake roller 6, and moves in a predetermined direction. An input shaft 82 of the powder clutch 74 is connected to an output shaft 80 of a speed reducer 79 through a coupling 76. An input shaft (not shown) of the speed reducer 79 is connected to an output shaft (not shown) of a DC motor 78 which is used for feeding the electrode wire 2.
The powder clutch 74 is a commercially available means for transmitting a torque, which comprises magnetic powder and a exciting coil. When the exciting coil is energized, it generates magnetic field. In this magnetic field, the magnetic powder connects the input shaft 82 to the output shaft 75 so that torque is transmitted from the input shaft 82 to the output shaft 75. The magnetic powder allows slip between the input shaft 82 and the output shaft 75. The torque transmitted from the input shaft 82 to the output shaft 75 is controlled by the current to the exciting coil which controls the slip between the two shafts. During the normal machining operation of the electrical discharge machine, the braking torque on the brake roller 6 produces the tension in the electrode wire 2.
When the electrical discharge machine begins to operate or when the electrode wire 2 is broken, an electrode wire 2 must be installed, that is, a new portion of the electrode wire 2 must be drawn from the wire bobbin (not shown) and be engaged with the wire conveying device provided in the lower wire guide or within a column. This process is referred to as "wire connection".
The wire tension setting device is also used during wire connection. The DC motor 78 is energized after the new portion of the electrode wire 2 is drawn from the wire bobbin and wound around the brake roller 6.
The rotation of the DC motor 78 is transmitted to the input shaft 82 of the powder clutch 74 through the coupling 76. When current is supplied to the powder clutch 74, the input shaft 84 is connected to the output shaft 75. Thus, the rotation of the DC motor is transmitted to the brake roller through the output shaft 75. The rotation of the brake roller 6 feeds the electrode wire 2 toward the wire conveying device. The above described wire connection is referred as "automatic wire connection", since it is automatically performed as one of the functions of the electrical discharge machine.
Once wire connection is completed, the current to the DC motor 7 is cut off. Since the electrode wire 2 is wound around the brake roller 6, the brake roller 6 continues rotating due to the friction between the surface of the brake roller 6 and the moving electrode wire 2. From FIG. 3, it may be understood that the powder clutch 74 applies the braking torque to the brake roller 6 with slipping between the input shaft 82 and the output shaft 75 when the DC motor 78 is de-energized because the input shaft 82 of the powder clutch 74 is prevented from rotating by the speed reducer 79. The current supplied to the powder clutch 74 is controlled so as to generate the proper braking torque. The braking torque applies tension to the electrode wire 2.
It may be understood from the above description that the wire tension setting device shown in FIG. 3 comprises the powder clutch 74 and the DC motor 78 connected in line relation to each other so as to perform both the wire feeding function and the wire tension setting function.
However, the wire tension setting device of the prior art shown in FIG. 3 has a disadvantage in that automatic wire connection cannot be carried out satisfactorily. When the electrode wire 2 is fed by the DC motor 78 for automatic wire connection, the input shaft 82 of the powder clutch 74 must be connected to the output shaft 75 in order to transmit the rotation of the DC motor 78 to the brake roller 6. The braking torque acts as a resistance to a force generated by the wire conveying device for drawing the electrode wire 2. This interferes with automatic wire connection.
In order to overcome this problem, some wire-cut electrical discharge machines comprise a wire feeder with a DC motor, and an independent wire tension setting device with a powder clutch. This type of wire-cut electrical discharge machine, however, has a disadvantage in that the machine requires two independent devices, that is, the wire feeding device and the wire tension setting device, whereby the cost is increased.
Furthermore, the wire tension setting device disclosed in the Japanese Unexamined Patent Publication Kokai) No. 63-300823 comprises a high torque electro-magnetic brake 15 and a low torque electro-magnetic brake 13 so as to make it possible to provide a wide range of wire tensions (see FIG. 1 in the above publication). However, the publication does not disclose the relationship between the electric motor and the electro-magnetic brake. Furthermore, the low torque electro-magnetic brake is a means for applying the proper torque to a thin electrode wire, and is not for connecting the electrode wire effectively.
Futhermore, some wire conveying devices have been improved so as to increase the force for drawing the electrode wire while using the type of the wire tension setting device shown in FIG. 3.
For example, a wire conveying device comprises an engaging roller, which is rotated by machining liquid directed onto the engaging roller in the form of jet, for drawing the electrode wire into the lower wire guide. Another wire conveying device is formed so as to direct the machining liquid in the form of jet into an arm member of the lower wire guide, and the electrode wire is drawn into the lower wire guide by the flow of the machining liquid. However, the above two devices do not increase the drawing force.
It is conceivable that in order to increase the drawing force sufficiently, a wire conveying device may comprise an electric motor and an engaging roller connected to the motor. However, in the recent mainstream of electrical discharge machine design, the workpiece and at least a portion of the wire conveying device are immersed in the machining liquid. Therefore, it is difficult to use a conventional electrical driving means in the wire conveying device in the lower wire guide.
For that reason, a wire-cut electrical discharge machine comprises an electric motor within the column thereof, and the electric motor rotates an engaging roller through a belt therebetween. This type of wire conveying device can generate a sufficiently large force compared with the braking torque by the powder clutch 74. However, in this manner, the belt must be led into a sump for machining liquid. This requires a complex sealing arrangement for the machining liquid.